Tungsten plugs are becoming a widely used form of making electrical connections between various conductive layers in semiconductor devices such as integrated circuits. Generally, tungsten plugs are formed by first depositing an interlayer dielectric over a first conductive layer of a semiconductor device. Openings are then etched in the interlayer dielectric to expose portions of the first conductive layer. Next, a tungsten layer is blanketly deposited over the device, thereby filling the openings in the interlayer dielectric and making contact to the first conductive layer. The tungsten is then either polished or etched back to remove all portions of the tungsten layer but for the portions lying within the opening of the interlayer dielectric. The result is a tungsten plug which serves as a vertical electrical contact to the first conductive layer.
One problem with the basic process described above is that tungsten does not readily adhere to commonly used interlayer dielectric materials such as silicon dioxide, in the form of phospho-silicate-glass (PSG), boron-doped PSG (BPSG), thermal-oxide, or plasma-enhanced oxide, and silicon nitride. Accordingly, use of tungsten plugs has generally required the use of a glue layer to ensure the tungsten adheres to the device. A typical use of a glue layer is demonstrated in FIGS. 1-2, which are illustrations representing a cross-section of a portion of a prior art semiconductor device 10. As shown in FIG. 1, semiconductor device 10 includes a semiconductor substrate 12 over which is formed a first dielectric layer 14 and a metal interconnect 16. An interlayer dielectric 18 is deposited, patterned, and etched to create a contact opening or plug opening 19 which exposes the interconnect. A glue layer 20 is then deposited on top of the interlayer dielectric and along the sidewalls and the bottom of opening 19. Glue layer 20 may be a single material, for instance titanium nitride, but is often a combination of sequentially deposited titanium and titanium nitride layers, depending upon to what conductive material contact is being made. Next, a tungsten layer 22 is blanketly deposited on the glue layer of device 10 to fill opening 19, as shown in FIG. 1. To provide electrical isolation between a plurality of filled openings with the device, the tungsten layer and portions of glue layer 20 lying beyond each opening 19 are removed, as indicated by FIG. 2. Removal of both glue layer 2 and tungsten layer 22 is generally accomplished either by etching or by polishing. The result is a tungsten plug 24 formed in opening 19, and having a surrounding glue layer 20 separating the tungsten plug from interlayer dielectric 18.
Use of a titanium and titanium nitride glue layer in forming tungsten plugs as discussed above has several drawbacks. One problem is that throughput is rather slow. The initial titanium layer for glue layer 20 is generally deposited to a thickness of 400 angstroms (.ANG.) or 40 nanometers (nm) using a collimated sputtering deposition process. Collimated sputtering is preferred to ensure that the titanium gets deposited uniformly and to a sufficient thickness along both the sidewall and the bottom of the contact opening. Titanium nitride is likewise sputter deposited, generally to a thickness of 800.ANG. (80 nm). Sputter deposition, and particularly collimated sputtering, is a relatively slow process. For instance, the deposition time required to deposit the aforementioned glue layer thicknesses is approximately anywhere from 3 to 5 minutes per wafer using a 1.5:1 collimator and sequential in situ deposition of the titanium and titanium nitride layers. In addition to valuable manufacturing time spent on depositing the glue layers, removal of the glue layer beyond the contact opening is also quite slow. For example, over one-half the polishing time needed to remove both a tungsten layer 22 and a titanium and titanium nitride glue layer 20 is spent on removing the glue layer, even though the glue layer is about one-fifth (20%) the thickness of the tungsten layer.
Yet another problem of using a glue layer of titanium and titanium nitride relates to contact resistance. The titanium portion of the glue layer is generally used to improve the contact resistance between the tungsten plug and the metal interconnect, as opposed to using titanium nitride by itself as the glue layer. However, even with the addition of titanium as part of the glue layer, contact resistance can still be a problem if the metal interconnect is damaged or contaminated during formation of the contact opening in the interlayer dielectric. For instance, in forming the contact opening using fluorine-based etch chemistries, fluorine will react with the aluminum of the interconnect to form an aluminum oxyfluoride (AlO.sub.x F.sub.y) which degrades contact resistance.